1. Field of the Invention
The present invention relates to controlling power to a lighting load using solid state dimming technology.
2. Description of the Related Art
Continuous dimming of lighting is desirable for a number of reasons. It can change the atmosphere of a room or compensate for illuminance losses caused by lamp lumen depreciation, dirt effects, and other light loss factors. It is also desirable to adjust the illumination in response to varying natural light conditions.
Many types of control systems are known for continuous dimming of lighting. Phase control dimming, a commonly used system, generally employs a controllably conductive device (i.e., a solid state switch) to interrupt the flow of power to a load during a portion of each half cycle. Although gate turn off devices and bipolar and MOSFET transistors have found limited use in phase control systems, thyristors or, more specifically, triacs are preferred. In what follows, we will assume that a triac is being used as the solid state switch.
A triac normally has three terminals--a cathode, an anode, and a gate (or control terminal). Current may be injected into or drawn out of the gate to cause the triac to fire (i.e., switch from a non-conductive state to a conductive state). Once fired, a triac remains conductive, even in the absence of gate current, until the conducted current drops below a certain level known as the holding current, which is generally about 1/1000 of its maximum current rating. Below this holding current, the triac reverts back to its non-conductive state. A phase control system operates by firing the triac after a time (or "phase") delay after each zero crossing of power flow from the AC source. During the phase delay, the triac is nonconductive, and no current flows to the load. Once the triac is fired, current flows to the load until the next zero crossing. By varying the delay, one controls the average power provided to a load--the greater the delay, the lower the power to the load.
The phase delay is typically varied with an adjustable time delay circuit (more commonly referred to as a "firing circuit"), consisting of a series-connected resistor and capacitor, and a suitable breakover device, such as a diac, connected between the gate of a triac and the capacitor. Voltage applied across the resistor and capacitor forces current to flow through the resistor, charging the capacitor according to a time constant determined by the product of the resistance and capacitance. After a certain predictable time delay following each zero crossing, the capacitor voltage reaches a predetermined value and the diac breaks over. The capacitor discharges into the triac gate and fires the triac. The time delay can be varied by adjusting the resistance or capacitance.
One problem commonly encountered with phase control systems is the emission of electro-magnetic interference (EMI). For a given power, the strength and frequency of the EMI is related to the switching time. Slow switching results in weak, low frequency EMI. Triacs switch very rapidly, and, as a result, produce strong, high frequency EMI.
EMI can be divided into two types of emissions, conducted emissions and radiated emissions. Conducted emissions use the power line itself as a path for propagation. Generally, this emission is limited to that branch of the circuit to which the emitting device is connected, and is of concern only to the extent that it may interfere with other devices on that same brach.
Radiated emission refers to the electromagnetic energy emitted into the air. Radiated emissions can induce unwanted voltages in nearby circuits, such as a radio or television, causing annoying disruption or even malfunction. The primary radiating antenna in the case of a phase control system used to control power to a lamp is generally the wiring.
In most countries, there are EMI emission standards and limits that electrical equipment manufacturers must adhere to. In the U.S., The Federal Communications Commission (FCC) broadly regulates such equipment (FCC, Part 15 or 18), and the Food and Drug Administration (FDA) issues separate standards for devices used in hospitals.
There are basically two modes of attack for reducing EMI emission. The circuit may be redesigned to eliminate rapid switching or, more commonly, when rapid switching cannot be eliminated, a filter may be used to attenuate specific unwanted components (usually the higher frequencies). The first alternative is difficult or, more often, impossible to do. The second alternative, designing a filter, is in principle a ralatively simple exercise. However, implementing a satisfactory design requires much iteration, because success depends not only upon the calculated impedances, but upon the characteristics of each individual component. These include core losses, saturation, magnetostriction, and impedance variation with frequency.
Another unavoidable problem is that capacitors have self-inductance and inductors have self-capacitance; both, therefore, are self-resonant. Often, the self-resonant frequencies of filter circuit components, or component assemblies, are near enough to the strongest EMI frequency that the filters so created do not significantly attenuate, and may even enhance, unwanted high frequency components. In some instances, even lower frequencies may not be attenuated. Furthermore, components that are adequate for filtering EMI may dissipate excessive heat, or produce unacceptable levels of audible noise. Beyond this, the geometries of the filter layout and control system and the interaction between the two can be critical to successful EMI reduction, and the difficulties escalate rapidly as power increases.